Commonly used fasteners, such as tapping screws, bolts, nuts, washers, rivets, plugs, screws, and screw parts are typically made of structural carbon steel. They are produced by applying neutral-or carbo-quenching, thermal refining, and then anticorrosion treatment. Alternatively stainless steel products other than the above-mentioned carbon steel ones can be used for corrosion resistance. Although the market for stainless steel products is small because their cost performance and strength are inferior to those of carbon steel, the market is still expanding steadily.
Under such circumstances both the properties of corrosion resistance and mechanical strength tend to be demanded at the same time. For this reason, product such as screws, for example, of 18-8-stainless steel material which have been nitrided and hardened so as to improve deficient strength and scuffing resistance which are conventional weak points, and the like have appeared. However, the nitrided stainless steel products have some disadvantages such as corrosion of the nitrided surface in a short period. On the other hand, products such as bolts made of nickel alloy are generally used as high corrosion resistant material better than SUS in a field such as petrochemical plants.
However, just as SUS, the nickel alloy material has a low heat conductivity, which is one third of that of irons, and has problems that seizure, and scuffing phenomena (a state that seizure caused combination of parts) tend to occur. It also has a problem of inferior tightening property because the coefficient of skill friction is high and improvement of hardness by quenching is impossible. Nickel alloy material employed as material which is hard is generally carburized and nitrided, but cementation-hardening by elements such as carbon and nitrogen is difficult which is easy with iron material.
Accordingly, it is an object of the invention to provide nickel alloy products with their surfaces nitrided and hardened which have both high mechanical strength and scuffing resistance.